staging/lustre/ldlm: Correct itree_overlap_cb return type

As per interval_search() prototype, the callback should return
enum, not int.
This fixes correspondign sparse warning.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>